In the past, as an optical receiving device, there has been an optical receiving device including an adaptive equalizer that adaptively equalizes residual chromatic dispersion, polarization mode dispersion, or waveform distortion. There has been disclosed a technique in which, in order to optimize tap coefficients serving as parameters of an adaptive equalizer, a weighted center position of a filtering shape determined by the tap coefficients is calculated and the tap coefficients are shifted so that a difference between the calculation result and the center of the taps of the adaptive equalizer is minimized. Such a technique has been disclosed in, for example, Japanese Laid-open Patent Publication No. 2012-119923.
In some cases, owing to a contributing factor such as disturbance, polarization rotation, an inter-polarization delay difference (a differential group delay: DGD), or a difference in loss between polarized waves (a polarization dependent loss: PDL) occurs in an optical signal that travels through a transmission path. The inter-polarization delay difference appears as a difference in a temporal direction between tap coefficients of a finite impulse response (FIR) filter, in other words, a difference in tap number. Therefore, in a case where the inter-polarization delay difference is large, tap coefficients whose weights are large occur in both of a small tap number and a large tap number, in some cases.
In addition, the difference in loss between polarized waves appears as a difference in weight between the tap coefficients. Therefore, in a case where the difference in loss between polarized waves is large, a difference occurs between the weight of a tap coefficient whose tap number is small and the weight of a tap coefficient whose tap number is large, in some cases. In that case, if the weighted center position of a filtering shape determined by the tap coefficients is obtained by dividing the sum of values by the sum of the weights of the tap coefficients, the values being obtained by multiplying the tap numbers by the weights of the tap coefficients, the obtained weighted center position is shifted to a side on which a tap coefficient whose weight is larger is located, compared with an expectation value.
In a technique of the related art, the tap coefficients are shifted so that a difference between the weighted center position and the center of taps is minimized. Therefore, if the weighted center position is different from the expectation value, some of tap coefficients having weights are discarded, in some cases. In other words, in a case where the inter-polarization delay difference and the difference in loss between polarized waves occur, it is difficult to generate tap coefficients for compensating the inter-polarization delay difference and the difference in loss between polarized waves, in some cases. In that case, since it is difficult to obtain parameters of the adaptive equalizer, which are capable of adapting to a disturbance at the time of communication, it becomes difficult to expect to further improve the reception performance of an optical receiver.